diff --git a/OJ notes/pages/Leetcode Combinations.md b/OJ notes/pages/Leetcode Combinations.md
index de5d035..bb76002 100644
--- a/OJ notes/pages/Leetcode Combinations.md	
+++ b/OJ notes/pages/Leetcode Combinations.md	
@@ -24,12 +24,41 @@ tag:#backtrack
 ___
 ### Problem
 
+Given two integers `n` and `k`, return _all possible combinations of_ `k` _numbers out of the range_ `[1, n]`.
 
+You may return the answer in **any order**.
 
 #### Examples
 
+**Example 1:**
+
+```
+**Input:** n = 4, k = 2
+**Output:**
+[
+  [2,4],
+  [3,4],
+  [2,3],
+  [1,2],
+  [1,3],
+  [1,4],
+]
+```
+
+**Example 2:**
+
+```
+**Input:** n = 1, k = 1
+**Output:** [[1]]
+```
+
 #### Constraints
 
+**Constraints:**
+
+-   `1 <= n <= 20`
+-   `1 <= k <= n`
+
 ### Thoughts
 
 > [!summary]
@@ -40,4 +69,38 @@ explanation on the topic.
 
 In my eyes, backtracking means DFS-like recursion and searching in solving problems.
 
+First, it add a option in the combinations, then backtrack it
+And when it's done, it gets poped out and try another solution, and backtrack.
+
 ### Solution
+
+Backtracking
+```cpp
+class Solution {
+  void backtrack(vector<vector<int>> &ans, vector<int> &combs, int n, int nex,
+                 int k) {
+    if (k == 0) {
+      ans.push_back(combs);
+    } else {
+      // try different solutions
+      for (int i = nex; i <= n; i++) {
+        combs.push_back(i);
+
+        backtrack(ans, combs, n, i + 1, k - 1);
+
+        combs.pop_back();
+      }
+    }
+  }
+
+public:
+  vector<vector<int>> combine(int n, int k) {
+    vector<vector<int>> ans = {};
+    vector<int> combs = {};
+
+    backtrack(ans, combs, n, 1, k);
+
+    return ans;
+  }
+};
+```
diff --git a/OJ notes/pages/Leetcode Letter-Case-Combination.md b/OJ notes/pages/Leetcode Letter-Case-Combination.md
new file mode 100644
index 0000000..ce65882
--- /dev/null
+++ b/OJ notes/pages/Leetcode Letter-Case-Combination.md	
@@ -0,0 +1,58 @@
+# Leetcode Letter-Case-Combination
+
+#### 2022-07-20 14:19
+
+> ##### Algorithms:
+> #algorithm #backtrack 
+> ##### Difficulty:
+> #coding_problem #difficulty-medium
+> ##### Additional tags:
+> #leetcode 
+> ##### Revisions:
+> N/A
+
+##### Related topics:
+```expander
+tag:#backtrack
+```
+ 
+ 
+
+##### Links:
+- [Link to problem](https://leetcode.com/problems/letter-case-permutation/)
+___
+### Problem
+
+Given a string `s`, you can transform every letter individually to be lowercase or uppercase to create another string.
+
+Return _a list of all possible strings we could create_. Return the output in **any order**.
+
+#### Examples
+
+**Example 1:**
+
+```
+**Input:** s = "a1b2"
+**Output:** ["a1b2","a1B2","A1b2","A1B2"]
+```
+
+**Example 2:**
+
+```
+**Input:** s = "3z4"
+**Output:** ["3z4","3Z4"]
+```
+
+#### Constraints
+
+**Constraints:**
+
+-   `1 <= s.length <= 12`
+-   `s` consists of lowercase English letters, uppercase English letters, and digits.
+
+### Thoughts
+
+> [!summary]
+> This is a #backtrack problem.
+
+### Solution
diff --git a/OJ notes/pages/Leetcode Permutations.md b/OJ notes/pages/Leetcode Permutations.md
new file mode 100644
index 0000000..58682d3
--- /dev/null
+++ b/OJ notes/pages/Leetcode Permutations.md	
@@ -0,0 +1,108 @@
+# Leetcode Permutations
+
+#### 2022-07-20 14:16
+
+> ##### Algorithms:
+> #algorithm #backtrack 
+> ##### Difficulty:
+> #coding_problem #difficulty-medium 
+> ##### Additional tags:
+> #leetcode 
+> ##### Revisions:
+> N/A
+
+##### Related topics:
+```expander
+tag:#backtrack
+```
+ 
+ 
+
+##### Links:
+- [Link to problem](https://leetcode.com/problems/permutations/)
+___
+### Problem
+
+Given an array `nums` of distinct integers, return _all the possible permutations_. You can return the answer in **any order**.
+
+#### Examples
+
+**Example 1:**
+
+```
+**Input:** nums = [1,2,3]
+**Output:** [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]
+```
+
+**Example 2:**
+
+```
+**Input:** nums = [0,1]
+**Output:** [[0,1],[1,0]]
+```
+
+**Example 3:**
+
+```
+**Input:** nums = [1]
+**Output:** [[1]]
+```
+
+#### Constraints
+
+**Constraints:**
+
+-   `1 <= nums.length <= 6`
+-   `-10 <= nums[i] <= 10`
+-   All the integers of `nums` are **unique**.
+
+### Thoughts
+
+> [!summary]
+> This is a #backtrack problem
+
+Backtrack problem, see alse [[Leetcode Combinations]]
+
+### Solution
+
+```cpp
+class Solution {
+  void backtrack(vector<vector<int>> &ans, vector<int> &combs, bool unused[],
+                 vector<int> &nums, int n, int k) {
+    // if the end is hit, append it to ans.
+    if (k == 0) {
+      ans.push_back(combs);
+    } else {
+      // iterate over the unused vector
+      for (int i = 0; i < n; i++) {
+        if (!unused[i]) {
+          continue;
+        }
+        unused[i] = false;
+        combs.push_back(nums[i]);
+
+        backtrack(ans, combs, unused, nums, n, k - 1);
+
+        combs.pop_back();
+        unused[i] = true;
+      }
+    }
+  }
+
+public:
+  vector<vector<int>> permute(vector<int> &nums) {
+    // This can be modified from combinations
+    vector<vector<int>> ans = {};
+    vector<int> combs = {};
+    int size = nums.size();
+
+    bool unused[size];
+    for (int i = 0; i < size; i++) {
+      unused[i] = true;
+    }
+
+    backtrack(ans, combs, unused, nums, size, size);
+    return ans;
+  }
+};
+```
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